Thursday, August 18, 2022

La Niña may be heading for rare ‘triple dip’

But it’s actually quite hard for us in NZ to draw a straight line between La Niña conditions and what that means for our weather.
The more oomph a La Niña event has (how much warmer than usual sea surface temperatures are north of NZ), the more likely it is to have an impact on NZ. File photo

La Niña looks to be developing yet again  – what is known as a rare “triple dip” La Niña. The second one only just came to an end in June, but even as that was announced there were already headlines about it reforming again before the year is out, most likely this spring. If you think of the fire danger signs we see all over New Zealand then we’ve moved out of red and orange but we’re still on yellow/moderate risk. Officially this is known as being on a “La Niña watch” – meaning the ingredients are there and are expected to build back up again into an official La Niña.

What does this mean for NZ?

It’s important to remember that the past two La Niña events brought two things to NZ: isolated pockets of flooding and larger areas of drought. La Niña is measured at the equator, which is as far north from NZ as Antarctica is south of it. It’s important to keep that in mind whenever you see headlines about La Niña and how “badly” it may affect NZ. 

It’s actually quite hard for us in NZ to draw a straight line between La Niña conditions and what that means for our weather. For the past two La Niña events there was almost no mention of widespread drought risks from state forecasters, until the droughts had already arrived. 

The climate pattern known as La Niña – the cooling of surface ocean water along the tropical west coast of South America – is showing signs of reforming again before the year is out, probably in spring.

So if we’re halfway between the equator and Antarctica we may be more like a traffic island – sometimes we have that green light and are waving down that La Niña tropical moisture and warmth, then other times the light is red and we’re ushering in some colder Southern Ocean air or windy westerlies.

The strength of each La Niña event matters too.

The past two La Niña events I’d describe as “moderate” in strength. The more oomph a La Niña event has (how much warmer than usual sea surface temperatures are north of NZ), the more likely it is to have an impact on NZ. This may explain why the past two La Niña events were “sort of recognisable” here in NZ. We didn’t get quite as much of the tropical storms and rain events that one might associate with La Niña – but we did still get some severe pockets of flooding in both main islands. This next La Niña event looks to also be moderate.

So will the third La Niña be the same as the others?

Not necessarily. Every weather and climate event is as unique as a fingerprint. In recent months NZ has had the biggest weather/climate shift in half a decade, as high pressure clears away to allow big rainmakers to finally come back in. With this change it might open the doors a little more for sub-tropical/rain events or ex-cyclones later this spring/summer.  One thing is for sure: NZ is mostly two mountainous islands partially in the roaring forties … anything can happen here and we should always be prepared for northern storms, southern storms and big long dry highs. 

More: Phil Duncan is the director of WeatherWatch

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